1. Field of the Invention
The present invention generally relates to charge transfer and/or amplifying devices and more particularly to a charge transfer and/or amplifying device of low noise which can detect signal charges at high conversion efficiency.
2. Description of the Prior Art
Various methods have been proposed so far to detect signal charges transferred by a charge-coupled device (CCD) and in this case, a floating diffusion amplifier (FDA) is frequently utilized for this purpose. The FDA, however, has the disadvantage such that, when a floating diffusion region is preset to a predetermined potential by a pre-charge gate, a large reset noise is produced. Also, in addition to the reset noise, a low frequency (1/f) noise, white noise and so on of a metal-oxide semiconductor (MOS) transistor exist and these noises are too large to be neglected.
For this reason, a video camera or the like using a CCD employs a correlation double sampling (CDS) circuit in order to reduce the noise. In this case, if a conversion efficiency is enhanced, then the reset noise becomes large. There is then the disadvantage that the large reset noise cannot be removed fully by the above CDS circuit.
Further, since the apparatus corresponding to a high definition television receiver (HDTV) system is operable at higher speed, a time enough for the CDS processing cannot be afforded, which unavoidably causes the reset noise or the like to be increased.
Therefore, it is proposed to remove the reset noise by improving the FDA so as to cope with the HDTV system, thereby enhancing the conversion efficiency. One of such proposals is reported in "High sensitivity charge detector for CCD" pp. 51 to 52.2-12 of preliminary article for national conference held by the Institute of Television Engineers of Japan on 1988, and also another proposal is reported in "ring-junction gate type low noise CCD charge detector" pp. 27 to 28.2-8 of preliminary article for national conference held by the Institute of Television Engineers of Japan on 1989.
According to the former proposal, a p-type channel of a detecting transistor is formed beneath a CCD buried channel region, whereby signal charges are transferred to the direction vertical to the channel of the detecting transistor by the buried channel region. Accordingly, a current flowing through the p-type channel of the detecting transistor is modulated by the transferred signal charge so that the signal charge can be detected from the above current.
In accordance with the latter proposal, a junction gate type field effect transistor (FET) employing a p-type well as a channel is disposed between an output gate and a reset gate, which is used as a driving transistor of a first stage source-follower amplifier. Since the gate of the junction gate type FET is the ring junction gate (RJG) unitarily formed with the charge detection capacitance, this transfer charge amplifying device is called a ring junction gate CCD charge detector. According to the principle of the transfer charge amplifying device, signal charges transferred through the CCD are accumulated in the junction gate and a drain current of the above-mentioned junction gate FET is modulated by the resultant potential fluctuation and the modulated drain current is transmitted to the source follower amplifier of the next stage.
The above-mentioned transfer charge amplifying devices of the improved type have the advantages such that the reset noise can be removed because they can discharge the signal charges fully by the complete transfer process. However, a hole current, which is a modulated current, is passed through the buried channel region and the depth of the buried channel region through which the hole current is passed is so large from the surface of the semiconductor substrate that a short-channel effect is large. Therefore, the mutual conductance (gm) cannot be increased and as a result the conversion efficiency cannot be enhanced. In this case, if the conversion efficiency is unreasonably enhanced, then the operation speed is decreased.